Lately, many researchers are studying about an Orthogonal Frequency Division Multiple Access (OFDMA) scheme and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme in progress to develop a method for transmitting data through a radio channel at a high speed.
OFDMA is a scheme for transmitting data using multi-carrier. That is, OFDMA receives a serial symbol sequence and modulators the received serial symbol sequence to a plurality of sub-carriers having orthogonality by converting the serial symbol sequence to parallel data.
FIG. 1 is a block diagram illustrating a transmitter of an OFDMA system in accordance with the related art.
Referring to FIG. 1, the OFDMA transmitter includes an encoder 11, a modulator 12, a serial to parallel (S/P) converter 13, an N sized inverse fast fourier transform (IFFT) processor 14, a parallel to serial (P/S) converter 15, and a cyclic prefix (CP) inserter 16.
The encoder 11 performs a channel encoding process. That is, the encoder 11 receives sequences of information bits and performs the channel encoding process on the received sequences. In general, a convolutional encoder, a turbo encoder, or a Low Density Parity Check (LDPC) encoder is used as the encoder 11.
The modulator 12 performs a modulation process based on a quadrature phase shift keying scheme (QPSK), 8 PSK, 16-ary quadrature amplitude modulation (16 QAM), 64 QAM, or 256 QAM.
The S/P converter 13 receives the modulated data from the modulator 12 and converts the received data to parallel data. The IFFT processor 14 receives the parallel data from the S/P converter 13 and performs the IFFT process on the received parallel data. The P/S converter 15 converts the output from the IFFT processor 14 to serial data. The CP inserter 16 inserts a cyclic prefix to the output data of the P/S converter 15.
As described above, the CP interval must be greater than a delay extension interval in the OFDMA system. However, it is not simple to decide a proper CP interval in consideration of various environmental factors in a wireless communication system. In the wireless communication environment, the delay extension usually has a small value. However, the delay extension may rarely have a great value if a terminal is located at a cell boundary, if a terminal is surrounded by mountains, or if delay is added by repeaters. The delay extension is not a unique variable of a cell. The delay extension may have different value according to the location of a terminal.
Since the delay extension has a small value in an urban area where needs a wideband communication system, a guard interval must have a small value in order to maximize the performance of a system. Here, a terminal having a great delay extension requires to process interference between symbols, which is generated when the delay extension exceeds the CP interval.
In a downlink of the OFDMA system, only a delay extension is considered for a CP interval. However, a guard interval must be also considered based on a timing error of an uplink signal as well as the delay extension. Particularly, a timing error may be great when an initial access process is performed in a large cell, when a handover process is performed in a large cell, or when a terminal does not exchange data with a base station for a long time. The timing error requires a very large guard interval because the timing error is added with the delay extension.
If the guard interval is not large enough, the performance of a system generally deteriorates due to interference between symbols. If the power is not controlled properly, the interference of a terminal can influence the performances of the other terminals. Radio resources may be seriously wasted if a CP interval is decided based on both of the maximum delay extension and the maximum timing error. Therefore, there is a demand for developing a method for individually maximizing a guard interval for a user terminal that generates a long timing error or long delay extension for sustaining a small CP interval for an uplink.